The present invention mainly relates to a radio transmitter of the TDMA (Time Division Multiple Access) system and, more particularly, to a transmission output control circuit for controlling an output level of a burst-shaped transmission wave and leading and trailing characteristics of an output for a wide dynamic range.
Hitherto, what is called an automatic power control (APC) circuit as shown in FIG. 1 has been known as such a kind of transmission output control circuit. Such an APC circuit functions so as to keep a transmission output constant for fluctuations of various conditions such as power supply voltage, input, temperature, and the like. In FIG. 1, reference numeral 1 denotes a transmission VCO (Voltage Controlled Oscillator); 2 a transmission power amplifying circuit; 3 a directional coupler; 4 a detecting circuit using a diode; and 5 a comparison error amplifier. An output from the transmission VCO 1 is power amplified by the transmission power amplifying circuit 2 and is transmitted through the directional coupler 3. The directional coupler 3 branches a part of the output of the amplifying circuit 2 and supplies it to the detecting circuit 4. A detection output of the detecting circuit 4 is supplied to the comparison error amplifier 5. The amplifier 5 compares the received detection output and a reference waveform 8 (V.sub.ref) and gives a control voltage V.sub.APC to the amplifying circuit 2 in accordance with the result of the comparison, thereby controlling an amplification factor. A transmission output control negative feedback loop is constructed as mentioned above. A transmission output level ordinarily has a discrete value. A difference between the maximum and minimum levels is up to 20 dB or slightly higher. Therefore, an object of such a loop can be substantially accomplished by a simple detecting circuit using a diode which has generally been well known.
In the narrow band TDMA radio communication which will generally be wide-spread in the future, however, since a transmission wave has a burst-like shape, the leading and trailing characteristics of the transmission wave must be accurately controlled in order to suppress a spread of the spectrum to an adjacent channel.
For instance, in the GSM standard as a Pan European integrated standard of the digital cellular telephone, the leading and trailing response time characteristics are set by specifying an output lower limit of -36 dBm or a point of a level relative ratio of -70 dBC to a peak value and a point of -30 dBC and a time which is required to reach the peak value.
However, since the detecting characteristics of the diode are applied to the general detecting circuit as mentioned above, the dynamic range is naturally narrow and the dynamic range is set to merely up to 20 at most and a few dB and it is considerably short of controllable transmission output ranges necessary for practical applications.